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Re: How does mysql work ?
Posted by: Rick James
Date: February 12, 2014 11:46AM

> Error on delete of '/tmp/#sql_7bd62_0.MYD' (Errcode: 2)

That is a temp file (table) used in support of a complex SELECT. Let's see the SELECT.

True, the file should not be deleted twice.

Check the permissions on /tmp. It should include "s".

> The whole process. from query to results.

That's a long discussion. And there are many possible paths it could take. If you provide a SELECT and the SHOW CREATE TABLE for the table(s) involved and the output of EXPLAIN on that SELECT, we can walk you through that example.

Possible paths involve:
* InnoDB vs MyISAM vs MEMORY
* JOIN (or not)
* UNION (or not)
* PARTITIONing (especially 'pruning')
* Subqueries (correlated, uncorrelated, etc)
* GROUP BY, ORDER BY, HAVING, LIMIT, EXISTS
* Indexes, PRIMARY KEY, clustered index, index-only, "index merge", MBR, ICP, etc.

If you want the "whole process", then see the online manual, the performanceblog site, comments in this forum, and many other places.

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